1. FIELD OF THE INVENTION
This invention relates to a process for purifying hydrogen fluoride. More specifically but not by way of limitation, the invention relates to the removal of trivalent arsenic (As.sup.+3) from anhydrous HF by precipitation of As.sub.2 S.sub.3.
2. DESCRIPTION OF THE PRIOR ART
Anhydrous hydrogen fluoride is commercially manufactured by heating a mixture of fluorspar (a naturally occurring calcium fluoride) and sulfuric acid. The main impurities that result from such a reaction (fluorosilicic acid, silicon tetrafluoride, sulfur dioxide, sulfuric acid and water) are usually removed by fractional distillation. The resulting hydrogen fluoride has a purity of about 99.8% or better. However, hydrogen fluoride thus produced usually also contains minor amounts of certain other impurities which includes arsenic. The degree to which this impurity is present in commercial anhydrous hydrogen fluoride depends largely on the source of fluorspar. Typically, arsenic may be present at levels of from about 50 parts per million (ppm) to about 1500 ppm, again depending upon the particular source of fluorspar.
The degree of purity of anhydrous hydrogen fluoride required is to a great extent dependent on the particular end use application. Thus, it is generally known that, for such applications as found in the electronics industry such as cleaning agents and etchants in the production of semiconductors, diodes and transistors, a high degree of purity and extremely low levels of impurities are required. Typically, arsenic concentrations in terms of a few parts per billion (ppb) are desirable. Thus, the prior art discloses several anhydrous hydrogen fluoride purification processes intended to reduce the arsenic concentration to levels measured in terms of ppb. However, these known processes are characterized as involving a combination of costly reagents, equipment and/or procedures, as well as frequently requiring prolonged periods of processing time.
For example, in U.S. Pat. No. 3,687,622 the distillation of impure anhydrous hydrogen fluoride containing 1200 ppm As at very high pressures (e.g., &gt;115 psia and preferably &gt;165 psia) is disclosed wherein the As is removed overhead, and purified hydrogen fluoride (e.g., &lt;3,000 ppb and preferably &lt;100 ppb As) is recovered as bottoms product. In U.S. Pat. No. 3,663,382 As impurities are removed from anhydrous hydrogen fluoride by distillation at a pressure below 25 psia, with purified hydrogen fluoride being recovered as the overhead product.
Most of the processes disclosed in the art to reduce arsenic levels in anhydrous hydrogen fluoride involve oxidation of As.sup.+3 to its pentavalent state (As.sup.+5) thereby to reduce its volatility and solubility.
For example, U.S. Pat. No. 4,032,621 discloses a process for purifying anhydrous hydrogen fluoride by treating anhydrous hydrogen fluoride sequentially with an oxidizing agent, such as alkali metal permanganate or alkali metal dichromate, and then with a metal-free reducing agent, such as sodium percarbonate or sodium perborate, and hydrogen peroxide followed by distillation. Reduction of arsenic level from 50 ppm to about 5 to 30 ppb is reported.
In U.S. Pat. No. 4,083,941 a process for purifying anhydrous hydrogen fluoride is disclosed which involves treating anhydrous hydrogen fluoride with persulfuric acid or hydrogen peroxide and then treating with methanol or sulfuric acid, followed by distillation. Reduction of arsenic levels from 25 ppm to 20 to 30 ppb is disclosed with treatment time in the range of 48 to 73 hours.
In U.S. Pat. No. 3,166,379 a process for removing arsenic from hydrogen fluoride is described wherein an oxidizing agent in combination with a halogen (iodine, bromine, or chlorine) is used to oxidize the impurities to high boiling point, oxidized impurities. The purified hydrogen fluoride is then recovered by distillation. In U.S. Pat. No. 4,491,570 a process for purifying anhydrous hydrogen fluoride by treating it with elemental chlorine and hydrogen chloride or a fluoride salt, followed by distillation, is described. Reduction of arsenic from 15 ppb to less than 0.5 ppb is disclosed. In U.S. Pat. No. 4,668,497 a process involving the addition of fluorine to oxidize impurities present in hydrogen fluoride, followed by distillation, is disclosed.
In U.S. Pat. No. 4,756,899 a process for purifying anhydrous hydrogen fluoride is disclosed wherein hydrogen peroxide, in the presence of molybdenum or a molybdenum compound and a phosphate compound, is used to oxidize the volatile As.sup.+3 to non-volatile As.sup.+5 followed by distillation. Reduction of As.sup.+3 from an initial range of 500 to 800 ppm to about 5 ppm in the treated hydrogen fluoride is reported.
In contrast to the known ultra-high-purity processes of the prior art, technical or industrial grade anhydrous hydrogen fluoride containing typically from about 50 to 100 ppm arsenic can usually be used in chemical processing or in the oil refining industry without too much difficulty. However, when the arsenic impurity level is higher, catalyst deactivation is usually accelerated, and at very high arsenic levels (e.g., from about 200 ppm to about 1500 ppm) corrosion of processing equipment also becomes very severe. For example, in the process of fluorinating chlorocarbons with hydrogen fluoride in the presence of antimony halide catalysts to produce fluorinated hydrocarbons, arsenic in the hydrogen fluoride will accumulate in the antimony halide catalysts, thus contributing to the accelerated deactivation of the catalysts. When the deactivated catalyst is reactivated or discarded, the presence of large amounts of arsenic in the spent antimony halide catalyst presents handling problems. The presence of large amounts of arsenic in the processing system can lead to greatly accelerated corrosion of process equipment if an oxidant, such as chlorine, is also present.
In the commercial manufacture of anhydrous hydrogen fluoride the technical grade hydrogen fluoride is purified by one or more final distillation steps. This conventional fractional distillation is effective in removing most of the major impurities, except for the arsenic impurities. Usual distillation procedures are ineffective in significantly reducing the arsenic level in anhydrous hydrogen fluoride since the arsenic is present in the trivalent (As.sup.+3) form, as arsenic trifluoride (AsF.sub.3), which will codistill with hydrogen fluoride. Consequently, there exists a need for a process that inexpensively and effectively decreases the arsenic impurities in anhydrous hydrogen fluoride to a level of less than about 100 ppm.